1. Field of the Invention
The present invention relates generally to a viscous fluid coupling device. In particular, the present invention is concerned with a preloaded control arm for controlling fluid flow in a viscous fluid fan clutch.
2. Statement of the Related Art
A thermostatically-controlled viscous fluid clutch assembly for driving and rotating a vehicle cooling fan is well-known. A multi-bladed fan is removably secured to a body of the clutch assembly. The fan and clutch assembly are installed between an accessory pulley of a vehicle engine (typically the water pump pulley) and a radiator. The clutch assembly drives the fan at high speeds close to input speed when cooling is required and permits the fan to rotate at low speeds when cooling is not required. Thermostatic control of the fan through the clutch assembly reduces the load on an engine and the noise caused by fan rotation, resulting in horsepower gain and improved fuel economy.
Generally, a clutch assembly includes a clutch plate having lands and grooves mated to the body having complementary lands and grooves. A pump plate separates a pair of internally-contained chambers, a collecting chamber and a pumping chamber, from a reservoir. Gates in the pump plate permit the flow of a viscous fluid from the reservoir to the collecting chamber and into a shear zone between the lands and grooves of the body and clutch plate. Fluid sheared between the lands and grooves transfers input torque from the clutch plate to drive the body and the attached fan.
Fluid flow through the pump plate gates is controlled by a control arm placed adjacent the pump plate. When cooling is not required, the control arm is rotated so that the gates are covered and the majority of the fluid in the shear zone is pumped into the pumping chamber. Orifices in the pump plate permit passage of the fluid from the pumping chamber into the reservoir. The removal of a majority of the fluid from the shear zone substantially reduces the shear between the clutch plate and the body, thereby substantially reducing the rotation of the fan.
When cooling is required, the control arm is rotated an opposite direction to uncover the gates and permit fluid to flow into the shear zone, thereby increasing the shearing force between the clutch plate and the body. This increase in input torque results in faster rotation of the fan to increase the flow of cooling air.
Oftentimes, a bimetallic element is utilized to actuate the control arm, and thus the flow of fluid through the pump plate gates. The bimetallic element may be connected to a control shaft which is in turn connected to the control arm. As the bimetallic element expands due to the temperature of warm ambient air, the shaft rotates, thereby causing the rotation of the control arm. As the ambient air cools, the bimetallic element contracts, causing the control shaft and the control arm to rotate in an opposite direction.
To accommodate manufacturing and assembly tolerances, and the expansion and contraction of a bimetallic element, the control shaft and the control arm are permitted to slide or "float" for small distances in conventional fan clutch assemblies. Such float or "end play" occurs along the longitudinal axis of the shaft. The end play of the control shaft permits greater machining and assembly tolerances, thereby reducing manufacturing costs. Furthermore, friction between the bimetallic element and a cover is reduced by the end play of the control shaft.
As the control shaft slides in a direction away from the pump plate, the attached control arm is pulled away from the pump plate and tends to wobble during rotation. When the control arm is in the closed position and pulled from the pump plate, leakage of fluid from the reservoir to the collecting chamber can occur at the gates. Unwanted fluid can continue to flow into the shear zone and create undesired high-speed rotation of the cooling fan.
The art continues to seek improvements. In current designs, it is desirable to provide tolerances for the sliding of the control shaft which connects the control arm and the bimetallic element. Concurrently, it is desirable to have an effective seal between the control arm and the pump plate when the gates are covered at all longitudinal positions of the control shaft and the attached control arm.